The present invention relates to the art of fluid monitoring and analysis. The invention finds application in conjunction with on-line (i.e., while in use) monitoring of highly electrically resistive fluids such as, e.g., lubricants, natural and/or synthetic motor oils, standard additives and/or adjuncts, combustion engine fuels, other hydrocarbon-based fluids used in transportation and industrial applications, and the like, and will be described with particular reference thereto. More specifically, the present invention relates to a method and apparatus for on-line analysis of a highly electrically resistive fluid""s quality and/or condition using a fluid""s electrical response, or change in a fluid""s electrical response to an applied AC signal to determine, e.g., the amount or depletion of performance additives, contamination with unwanted liquids or solids, general degradation of the fluid due to chemical breakdown, or other changes in the fluid""s condition or quality. However, the present invention is also amenable to other like applications.
It is understood as used herein, a highly electrically resistive fluid refers to a fluid that has bulk resistivity at 20xc2x0 C. greater than about 103 ohm-m, preferably greater than about 105 ohm-m and more preferably greater than about 106 ohm-m.
It is understood as used herein, xe2x80x9cACxe2x80x9d is used to refer to a voltage, that is, an electrical potential, that has a non-zero frequency; xe2x80x9cDC voltage offsetxe2x80x9d or xe2x80x9cDC offsetxe2x80x9d is used to refer to the time average value of an xe2x80x9cAC voltagexe2x80x9d; and xe2x80x9cAC signalxe2x80x9d is used to refer to a combination of AC voltages with DC offsets.
Highly electrically resistive fluids are a critical component for the proper operation of many devices and/or processes. For example: lubricants are needed for an internal combustion engine to efficiently provide power over a long service life; high quality fuel is needed for proper engine operation with minimal emissions; and metal working fluid is needed for rapid waste metal removal and maximum tool life. Optimum performance is achieved when the fluid in question is of proper quality for the application, that is, the fluid preferably includes an appropriate base fluid and proper performance additives, e.g., corrosion inhibitors, friction modifiers, dispersants, surfactants, detergents, and the like. During use or consumption, the condition of the fluid should remain within determined limits, that is, chemical and/or other changes to the fluid should be limited to ensure proper performance. Changes that can occur to a fluid during use are, e.g., oxidation of the base fluid, depletion of performance additives, build-up of contaminants from external sources and/or from breakdown of the fluid""s chemical components, and the like.
Often, device owners and/or process operators depend on suppliers to provide proper quality fluids, and depend on regular fluid maintenance to maintain proper fluid condition. However, the foregoing is inherently limited and does not provide protection against accidental fluid substitution, or catastrophic fluid failure. In addition, regularly timed maintenance intervals can be wasteful if a fluid, with remaining useful life, is prematurely replaced or refreshed. Such premature maintenance, however, is often desirable rather than risk damage or excessive wear due to overly degraded fluids. In any event, owners and/or operators can minimize fluid maintenance costs without risking damage or excessive wear if fluid maintenance occurs only at the end (natural or otherwise) of the fluid""s usefulness based on the monitored fluid condition. Hence, an on-line fluid monitoring method and apparatus is desired which achieves a substantially xe2x80x9creal-timexe2x80x9d determination of the fluid""s initial quality and of the fluid""s continuing condition during use.
Heretofore, achieving an appropriate fluid monitoring method and apparatus for many applications has been difficult due to one or more reasons. For example, typical transportation and industrial fluids are complex mixtures of base fluids and additives that, even without contaminants, do not lend themselves to easy analysis. Often, the fluids are used and/or consumed in a relatively harsh environment that is not suitable for some analytical equipment and methods. Additionally when implementing the method and/or apparatus, there are always cost constraints to consider, both initial and long term.
To satisfy the cost and environmental constraints associated with real-time on-line fluid quality and/or condition monitoring, methods that measure electrical properties of fluids offer significant advantage. For complex fluids, where multiple changes in fluid chemistry and composition can confound single-point electrical property measurements, multi-point techniques are used. Two conventional xe2x80x9cmulti-pointxe2x80x9d techniques that measure electrical properties of fluids are voltage-dependent electrochemical analysis and frequency-dependent Electro-Impedance Spectroscopy (EIS).
There are a variety of voltage based electrochemical fluid analysis techniques, e.g., voltammetric techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV), linear scan voltammetry (LSV), differential pulse voltammetry (DPV), and normal pulse voltammetry (NPV), and time based techniques such as modified chronoamperometry (MCA). Generally, in each of these techniques, a fixed or slowly varying DC voltage is applied between either two or three electrodes of an electrochemical cell and measurements of the resulting current are plotted as a function of voltage and/or time. Voltage based electrochemical techniques provide information about low-resistivity fluids. However, these techniques are, in general, not suitable for highly resistive fluids. The extremely low current levels produced in highly resistive fluids make analysis difficult, and for many fluids, non-conductive fluid components can coat the electrodes, thereby inhibiting meaningful analysis. Off-line, voltage-based electrochemical analysis of highly resistive fluids can be conducted with high-cost, high-sensitivity electronics that solve the low-current-level problems, and can utilize chemical separation of fluid components before analysis to solve the electrode-coating problem. The off-line equipment and methods are, however, unsuited to an on-line environment with real-time analysis. On the other hand, U.S. Pat. No. 5,518,590 to Fang discloses a voltage-based on-line electrochemical method and apparatus for fluid analysis that uses a cell with a conductive electrolyte liquid or gel-like interphase surrounding the electrodes to overcome limitations associated with highly resistive fluid. The Fang technique, however, suffers from the limited robustness of the specialized electrochemical cell, and consequently the technique does not lend itself to broad application.
Conventional frequency-dependent EIS, when applied to highly-electrically-resistive fluids, has been limited to applying an AC voltage with zero DC offset voltage, between two electrodes immersed in the fluid to be monitored. The applied AC voltage and resulting current are used to determine the fluid"" electrical impedance. By using a multitude of frequencies, for example two as disclosed in European Patent Application EP 1 014 082 A2, Bauer et al., filed December 1999, both the bulk impedance of the fluid and the electrochemical properties of the fluid at the surface of the electrodes can be studied. While EIS is relatively low cost and not affected by highly resistive fluids, conventional frequency-dependent EIS does not provide the level of detail regarding fluid quality and condition that voltage-dependent electrochemical techniques provide.
Accordingly, the present invention provides a new and improved highly-electrically-resistive-fluid monitoring apparatus and method that overcomes the above-referenced problems and others.
The present invention relates to a method of monitoring a highly electrically resistive fluid. The method includes the steps of applying an AC electrical potential across the fluid at a first frequency and a first DC offset such that a first electrical response results; measuring the resulting first electrical response; applying the AC electrical potential across the fluid at a second frequency for a non-zero first DC offset voltage, and/or a second DC offset such that a second electrical response results, the second frequency and the second DC offset being different from the first frequency and the first DC offset respectively; measuring the resulting second electrical response; and analyzing the fluid""s quality and/or condition from the measured first and second electrical responses to the respective first and second applied electrical potentials.
The method can further include repeatedly applying the AC potentials, repeatedly measuring the resulting electrical responses, and analyzing the quality and/or condition of the fluid using the measured first and second electrical responses and/or changes in the measured first and second electrical responses to the respective first and second applied electrical potentials.
The method can further include the step of controlling the applied AC potentials based on determined electrical impedance, analyzed fluid quality and/or condition if the AC potentials are repeatedly applied.
The method can further include measuring the fluid""s temperature.
The method can further include compensating the fluid quality and/or condition analysis for variations in fluid temperature.
The method can further include the step of heating the fluid to a desired temperature.
The method can further include the step of controlling the applied AC potentials based on measured temperature.
The method can further include the step of determining the quality of a refreshment fluid when either a complete replacement or a partial refreshment of the monitored fluid occurs.
In another aspect of the invention, the first and second electrical responses are currents resulting from the applied AC electrical potentials.
In another aspect of the invention, the fluid quality and/or condition can be analyzed using electrical impedance values determined from measured electrical responses corresponding to applied electrical potentials.
In another embodiment of the invention, the method includes the steps of applying across the highly electrically-resistive fluid an AC signal that includes at least two different AC electrical potentials with at least one AC electrical potential having a non-zero DC offset, measuring the fluid""s electrical response at each applied potential, and analyzing the quality and/or condition of the fluid using the applied AC signal and corresponding measured electrical responses.
The method can further include repeatedly applying the AC signal, repeatedly measuring the resulting electrical responses, and analyzing the quality and/or condition of the fluid using the applied AC signal and measured and/or changes in the measured corresponding electrical responses.
In another aspect of the invention, the AC signal can be AC electrical potentials where DC offset is held fixed and frequency is effectively swept from one frequency to another either in a continuous manner or in a series of discreet frequency steps for at least one non-zero DC offset.
In another aspect of the invention, the AC signal can be AC electrical potentials where frequency is held fixed and DC offset voltage is effectively swept from one DC offset voltage to another either in a continuous manner or in a series of discreet voltage steps for at least one frequency.
In accordance with another aspect, the present invention further includes a highly-electrically-resistive-fluid monitoring apparatus having at least a pair of separated electrodes that are immersed in a fluid being monitored; at least one signal generator that applies to the electrodes an electrical signal with at least two different AC potentials with at least one potential having a non-zero DC offset; at least one monitor that measures an electrical response to the applied signal; and a controller that analyzes applied electrical signal and corresponding measured electrical responses to determine the quality and/or condition of the fluid.
In another aspect of the invention the monitor(s) is a current sensor, which measures a current generated in response to the applied potentials.
In another aspect of the invention, the controller that analyzes the quality and/or condition of the fluid can control the signal generator.
In another aspect of the invention, the apparatus can further include a temperature sensor that monitors a temperature of the fluid.
In another aspect of the invention, the apparatus can further include means for compensating the fluid quality and/or condition analysis for variations in fluid temperature.
In another aspect of the invention, the apparatus can further include temperature control means for regulating the temperature of the fluid.
In another aspect of the invention, the apparatus can further includes means for controlling the signal generator(s) based on the monitored temperature of the fluid.
In another aspect of the invention, the apparatus can further include means for determining when the fluid being monitored is totally replaced.
In another aspect of the invention, the apparatus can further include means for determining when the fluid being monitored is partially refreshed and the concentration of the refreshment fluid.
In another aspect of the present invention, the apparatus for monitoring highly electrically-resistive fluids includes sensing means in contact with a fluid being monitored. Further included are signal generating means in electrical communication with the sensing means. The signal generating means apply to the sensing means electrical signal having AC potentials of selected frequencies and selected DC offsets. The frequencies are selected such that there are at least two different frequencies for a non-zero DC offset, and/or the DC offsets are selected such that there are at least two different DC offsets. Monitoring means measure electrical response to the electrical signal via the sensing means. Control means analyzes the quality and/or condition of the fluid using the applied electrical signals and corresponding measured electrical responses.
One advantage of the present invention is that both the AC and DC dependence of a highly electrically-resistive fluid""s electrochemical properties are analyzed.
Another advantage of the present invention is that the time required to determine the electrochemical detail can be optionally reduced by fluid heating.
Another advantage of the present invention is that the temperature dependant nature of the electrochemical measurements can be optionally compensated.
Another feature of the present invention is that the applied electrical signal can be optionally controlled based on the analyzed fluid quality, or fluid condition, and/or the monitored fluid temperature.
Another advantage of the present invention is that refreshment of the monitored fluid can be determined to allow analysis of the refreshment fluid""s quality.
Another advantage of the present invention is that the fluid analysis provided can include an analysis of a highly electrically-resistive fluid""s solution, bulk, charge transfer, electrochemical reaction properties and the like.
Another advantage of the present invention is its compatibility with on-line environments.
Still further advantages, features and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.